Download enough shares to re-assemble a CHK-encoded object

CHK uses ZFEC and a k of n encoding. Given a CHK capability, k shares are required for decoding. If k shares can be found on and downloaded from the known storage servers then they can be fed to the decoder along with the (a) verify cap to check them for correctness and (b) the read cap to reproduce the original plaintext.

So, implement an API which - given a list of servers and a CHK capability - downloads k shares, if possible, and makes both the share number and share bytes available to the caller.